camera renders to targetTexture and main screen?

I am trying to generate thumbnails of objects I have in unity with a separate camera. I leave the camera disabled so that it does not render to the screen and then use camera.Render() to manually take a snap shot after I place an object in front of it. The problem is that for a split second the thumbnail appears in the bottom left of the main screen even though I set the camera.targetTexture = new RenderTexture(size, size, 24). The image that flashes up has the same dimensions as the 'size' of my RT so I am pretty sure that this RT is being drawn to the main screen. Is there any way to get the camera to only render to the RT and not show it on the main screen?

Here is my code -

 protected Texture TakeScreenShot() {
     camera.targetTexture = renderTexture = new RenderTexture(size, size, 24);   // tell camera to render to renderTexture
     //RenderTexture.active = renderTexture;                        // does nothing so how ReadPixels works I don't know
     camera.Render();                        
     
     // texturize rendering
     Texture2D screenshot = new Texture2D(size, size, TextureFormat.RGB24, false);
 screenshot.ReadPixels(new Rect(0, 0, size, size), 0, 0);
     screenshot.Apply();
     
     camera.targetTexture = null;
     RenderTexture.active = null;
 
     // return shot
     return screenshot;
 }